LSD was found to be a strong antagonist of the constrictor action of 5-hydroxtryptamine (serotonin) on the pulmonary vessels of the cat and the hind leg vessels of the cat and dog, but had only a weak effect on the constrictor actions of adrenaline and noradrenaline. The blocking action of LSD towards 5-hydroxytryptine could not be demonstrated in the rabbit ear vessels because LSD itself had a constrictor action. Gynergen was less effective than LSD in modifying the vasoconstrictor action of 5-hydroxytryptamine on the pulmonary vessels of the cat.